1. Field of the Invention
The present invention relates to acquiring seismic acoustic data and more particularly to acquiring and processing multifold vertical seismic profile data in order to accurately image vertical interfaces, such as the flank of a salt dome.
2. Related Prior Art
Prior art has produced several methods that attempt to image the flank of a steeply dipping interface, such as that of a salt dome. However, there are serious limitations in the practical effectiveness of these techniques (i.e. a good salt flank image is rare).
Currently, vertical seismic profile (VSP) imaging of a salt flank is accomplished using either an offset geometry (single source, many receivers) or a walkby geometry (multiple sources, few receivers). The data is processed using a ray tracing Vertical Seismic Profile to Common Depth Point (VSP-CDP) transformation or using a special migration program. Both techniques require a detailed geologic model to accomplish the processing.
In addition, some methods to image nearly vertical interfaces require the use of both downhole sources and receivers when imaging vertical or nearly vertical interfaces. When sources and receivers are used together with multifold processing techniques, both sources and receivers may be located on the surface (conventional seismic data acquisition), or the source may be located on the surface and the receiver may be located below the surface in a wellbore (vertical seismic profile), or both the source and receiver may be located below the surface. Some examples are contained in U.S. Pat. Nos. 2,718,930 (Bazhaw), 4,627,036 (Wvatt) and 4,509,149 (Ruehle).
U.S. Pat. No. 2,718,930 titled "Method of Deep Well Surveying" (Bazhaw) outlines a borehole seismic technique designed to help delineate the flanks of a salt dome, reef or other nearly vertical reflector or interface. A string of geophone receivers is laid out within the borehole, and a seismic source is employed in the borehole adjacent the vertical seismometer spread. The seismic waves move laterally from the borehole to the salt and back to the receivers, imaging the salt surface.
U.S. Pat. No. 4,627,036 titled "Vertical Seismic Profiling" (Wvatt) relates to a technique in which vertical seismic profile data is transformed into surface seismic data by mapping segments of the VSP seismic data into surface seismic data time configuration. The transformation results in data which is more easily interpreted with respect to determining subsurface structure because the data is stacked and because the data is in a more conventional form. This technique transforms conventional Vertical Seismic Profile (VSP) data, acquired using a long offset source, into the equivalent of surface seismic data. A very large offset is required for this method to operate accurately. By mapping VSP data into equivalent surface seismic data, the multifold common-depth-point (CDP) procedure may be used to enhance data quality. This manipulation is often referred to as the VSPCDP stack.
U.S. Pat. No. 4,509,149 titled "Directional Long Array for Logging Vertical Boundaries" (Ruehle) relates to a logging sonde that has directivity control of multiple sources and receivers in linear arrays. Each of the sources produces acoustic pulses which are delayed by a time delay between the sources to direct acoustic energy at an angle such that the resulting acoustic wave strikes a vertical formation orthogonally. The reflections of the acoustic pulses are detected with a linear array of receivers. The acoustic pulse detected by each receiver is delayed by an amount such that the total differential moveout for a reflected directional wave is zero.
Methods which are demonstrative of multifold processing techniques which are currently in use in the art are U.S. Pat. Nos. 3,867,713 (Teoland et al.), 4,241,429 (Bloomquist et al.) and 4,467,460 (Johnson). U.S. Pat. No. 3,867,713 titled "Multifold Seismic Exploration Over Purposefully Crooked Traverses" (Teoland et al.) relates to a processing technique in which seismic impulses are generated successively at locations uniformly spaced along traverse segments which regularly cross a median line. The segments are of a length equal to an integral multiple of the spacing between the locations. Seismic waves resulting from each impulse are detected at a set of the locations. This set of locations bears the identical traverse relationship to its shot point where the impulse giving rise to the detected waves is generated as every other set of locations bears to its shot point. Preferably, the detected signals are stacked on a common depth point basis to portray regularly patterned multifold a real subsurface structure.
U.S. Pat. No. 4,241,429 titled "Velocity Determination and Stacking Process from Seismic Exploration of Three Dimensional Reflection Geometry" (Bloomquist et al.) relates to a method in which linear, multiple fold, common depth point sets of seismograms with three dimensional reflection geometry are used to determine the dip and strike of the subsurface reflecting interfaces and the average velocity of the path of the seismic energy to the reflecting interface. The reflections in each set appear with time differences on a hyperbola with trace spacings determined by the source receiver coordinate distance along the lines of exploration. The offset of the apex of this hyperbola is determined from a normal move-out velocity search of the type performed on two dimensional CDP sets. This search identified the correct stacking velocity and hyperbola offset which are used to determine dip, strike and average velocity.
U.S. Pat. No. 4,467,460 titled "Seismic Data Acquisition Method" (Johnson) relates to a method for locating seismic acoustic pulse sources to simplify multifold processing. This method is primarily concerned with coupling that occurs in reception of acoustic pulses by geophones or other acoustic pulse receivers. The field locations of seismic shot points are chosen to produce partial multifold data, the static correction equations of which are at least partially coupled. The seismic cross sections resulting therefrom are substantially improved.